Incurable infectious neurodegenerative diseases, such as "mad cow" disease and human Creutzefeldt-Jacob disease, are postulated to be transmitted by an abnormal aggregation-prone protein isoform (prion). Prion aggregates are believed to be capable of seeding aggregation of the normal cellular protein, converting it into a prion. Pathology of prion diseases is reminiscent of other protein assembly disorders (such as Alzheimer?s disease) associated with amyloid-like aggregation. In yeast, prions control phenotypic traits inherited via cytoplasm. This provides a molecular basis for protein-based inheritance. As several unrelated proteins exhibit prion-forming potential, it is likely that protein-based inheritance may play an important biological role. The power of yeast genetic analysis makes yeast a useful model for studying general mechanisms of prion propagation. While many proteins can form amyloid-like aggregates in vitro, only some of them are capable of transmitting the aggregated state in vivo. This means that cells are normally able to prevent aggregate propagation. However, prions are overcoming the cellular defense systems. Moreover, our research has proven that the cellular stress-defense machinery becomes an essential component of prion propagation in the yeast cell. The overall, goal of this proposal is to uncover mechanisms by which cellular systems control prion formation and propagation in the yeast cell. This will explain how environmental and physiological factors induce protein-based inheritable variations, and may provide a new tool for curing aggregation-based disorders by altering the cellular regulatory systems. Proteins affecting prion formation and propagation in yeast will be investigated: stress-related chaperones of the evolutionary conserved Hsp100 and Hsp70 groups; ubiquitin system, normally involved in targeting misfolded proteins for degradation; and cytoskeletal assembly proteins involved in formation of cortical actin patches and endocytic vesicles. Specific Aims of the proposal are as follows: 1) To study a mechanism of prion regulation by the heat shock protein balance. 2) To study the role of ubiquitin system in the cellular control of yeast prions. 3) To study interactions between prions and cytoskeleton-associated structures in yeast.